This invention relates to impeders for use in induction welding. It is especially useful in the welding of tubing of small inner diameter, where the limited space inside the tubing makes cooling of the impeder difficult.
When high frequency induction welding is used to weld small diameter tube formed from strip, an impeder must be used inside the tube to prevent the current from flowing around the tube and overheating the whole tube. The ferrite rod that impedes the flow of current around the tube is usually cooled by flowthrough water or liquid nitrogen. In some environments, it is not convenient to have water flow through in only one direction, and liquid nitrogen is expensive, so there is a need for return water flow impeders. Such impeders have been made and used successfully heretofore in tubing having inside diameters of three-fourths inch and larger, but not successfully in smaller tubing. The present invention overcomes the problems of frequent breakage, inadequate cooling, etc. that have prevented the successful use of return flow water cooled impeders in the induction welding of tubing having inner diameters in the order of five-eights inch (16 millimeters) and less.
Impeders are provided in radio frequency welding of continuous metal strip to maximize the efficiency and line speed of the operation. The impeder concentrates the magnetic flux field set up by the high frequency induction coil, and thereby impedes the induced eddy current flow in portions of the tubular metal shape in which heating is not desired. By increasing the impedance of such extraneous paths, the impeder causes a concentration of the induced current in those portions of the moving tubular metal where heating is desired for attaining welding temperatures. Therewith, the excess sidewall heating of the tubular metal shape is reduced, and the eddy current flow induced into the tubular formed metal for purposes of heating the edges thereof to welding temperature is caused to flow mainly on the outside surface of the metal and along the edge portions of the tubular formed strip as well as through the apex of the "V" formed by the converging edge portions.
Various metals can be welded at high speeds with the welding apparatus of the present invention. Blackplate, tinplate, and aluminum and aluminum-magnesium alloys, in both light-gauge (0.002-0.004 inch thickness) and heavier gauges, may be welded by lapp or butt welds with increased speeds and decreased power requirements by utilizing the impeder construction of the present invention. Steel has been welded in this range down to 0.002 inch while aluminum has been welded in thicknesses down to 0.004 inch.
Cores of ferromagnetic material have been used as impeders in continuous welding lines. The impeder member presents the problem of difficulty in cooling. The sidewall temperature of the welded material often attains extremely high temperatures, indicating severe power loss. The temperature of the impeder member is only with difficulty maintained below the Curie temperature. Above the Curie temperature the phenomena of ferromagnetism disappear and the impeder substance becomes merely paramagnetic; then acting as if it were not present. Such heating cannot be tolerated in tube welding apparatus.
Impeder members of ferrimagnetic material have been employed in various arrangements for induction coil welding, but because of the limitations, such as overheating and low machinability characteristics, the use of such impeder members has been restricted. A class of such ferrimagnetic material is that termed as ferrites, which are a sintered ceramic mixture of iron oxide and various complex oxides of other metals. Such materials must be of low electrical conductivity, high saturation flux density, and high magnetic permeability; and they should have high thermal conductivity and relatively high Curie temperature.
The impeder of the present invention provides a low reluctance path for the leakage magnetic flux generated by the induction coil. The rapidly alternating flux produces a voltage in the impeder member which causes a current to flow. This induced current produces first, heat in the impeder, and secondly, its own field, which has the effect of distorting the magnetic field of the induction coil, according to Lenz's law. For these reasons it is essential that the impeder members be constructed of a material having low electrical conductivity and high magnetic permeability.
The problems mentioned above become more difficult, and the requirements become more stringent, with smaller tubing. The problem of cooling the impeder adequately is especially difficult for tubing about 16 millimeters and smaller in inside diameter; and the present invention is particularly advantageous in its overcoming of this problem.
Accordingly, it is the primary object of the present invention to provide an improved impeder which is especially useful in induction welding of small diameter tubing on the order of 16 millimeters (I.D.) and less.
It is another object of the present invention to provide an improved return liquid flow impeder adapted to prevent the magnetic impeder member from overheating even in induction welding of small diameter tubing.
In addition, it is an object of the present invention to provide an improved return liquid flow impeder which is generally self-centering under the force of the liquid flow, and contains no spacers in an outer liquid heat exchange passage which would obstruct liquid flow.
It is a further object of the present invention to provide an improved return liquid flow impeder which includes a flexible connection between an enclosure member for the impeder and an outer feed tube for providing a liquid-tight seal and minimizing the possibility of breakage.
Additional objects and advantages of the present invention will become apparent from a reading of the detailed description of the preferred embodiment which makes reference to the following set of drawings.